Pigment paste resins are an important component of cathodically depositable coating compositions and exert substantial influence on the bath parameters and on physical and chemical characteristics of the paint films.
Pigment paste resins based on epoxy resins are disclosed in U.S. Pat. No. 3,936,405 and U.S. Pat. No. 4,035,275. These patents are directed to cationic water-thinnable polymers which are obtained by the reaction of polymers containing 1,2-epoxide groups, especially of polyglycol-modified epoxy resins, with amine salts and with mixtures of phosphine acids and sulphide acids giving rise to onium groups.
Similar products to those obtained in U.S. Pat. Nos. 3,936,405 and 4,035,275 which are crosslinked by transurethanization are disclosed in DE-PS 2,634,211, DE-OS 2,634,229, or DE-OS 2,928,769. Pigment wetting of resins crosslinkable by transesterification can be enhanced by the introduction of quaternary ammonium groups, and their use as pigment grinding resins is thereby made possible. This variant is disclosed, for example, in DE-OS 3,322,781. To improve the stability of these pigment pastes, DE-OS 3,622,710 suggests formation of quaternary ammonium groups in the presence of the pigment powder.
Pigment paste resins which yield coatings with improved corrosion resistance are obtained, according to EP-Al-0,107,088, by reacting monoepoxides, such as alkylene oxides, glycidyl esters or glycidyl ethers, with long-chain tertiary amines in the presence of an acid and/or water with the formation of quaternary ammonium groups. According to EP-Al-0,107,098, higher pigment loading is possible if these systems additionally contain amide groups. EP-B1-0,107,089 discloses that polyepoxides can be reacted in the same manner whereby processing using higher proportions of pigments is made possible.
Pigment paste resins based on alkylphenol-modified epoxy resins containing sulphonium groups, such as those disclosed in EP-A2-0,251,772, are alleged to permit formation of thicker films.
In another type of a pigment paste resin disclosed in EP-Al-0,199,473, an epoxidized polybutadiene is reacted with a secondary amine, and the addition of an acid and a monoepoxide leads to the introduction of quaternary ammonium groups.
Pigment paste resins based on epoxy resins, comprising oxazolidine groups, are disclosed, for example, in AT-PS 365,214 and AT-PS 380,264. Resins of this type exhibit good general characteristics and are commercially used in spite of having poor dispersibility characteristics in the bath material.
It is apparent from the voluminous literature in the field of pigment paste formulations for cathodically depositable coating compositions that this is a problem area for which there is still no completely satisfactory solution in spite of research which has extended over a number of years and in spite of numerous suggestions.